Transmitters are often used in pressure systems which conduct a fluid flow, e.g. in vacuum systems or in refrigeration systems.
Normally, the transmitters comprise a connection piece, for example with a screw threaded part or a welding flange for attachment to the pressure system. The connection piece typically forms part of a fluid communication conduit from the pressure system to a pressure sensor unit which is assembled hermetically with the connection piece. Normally, the sensor unit is inserted into the conduit to form a closure thereof, and subsequently it is fastened hermetically to the connection piece by welding. The transmitter may further have various electronic components for transforming the sensed pressure into a standardized electric signal.
In the known transmitters, the connection piece comprises a cylindrical cavity with a circular cross-sectional shape and with an exactly fixed radial size in its full depth. The sensor, correspondingly, has the shape of a piston of a dimension fitting into the cylindrical cavity. During the manufacturing of the transmitter, the sensor unit is pressed into the cavity and subsequently, the connection piece and the sensor unit is welded together from the outside. In order to establish a temporarily fixed connection to maintain the sensor unit in a fixed position in the connection piece during welding, the dimensions of the sensor unit and the cavity are very close to each other to form an interference fit, and the sensor unit therefore has to be pressed into the cavity by use of force. This complicates the assembling work and requires narrow tolerances and thus more expensive components. Moreover, it has been found that gases caused by the welding process can be confined in the material and potentially lead to leakage between the connection piece and the sensor. The confining of the gases is intensified by the interference fit between the connection piece and the sensor.